The present invention relates to improvements in ductile cast iron and its heat treatment.
The strength of cast iron has been improved remarkably by the development of spheroidal graphite cast iron, i.e. ductile cast iron, but its ductility and impact resistance are still behind those of the steel.
To improve these mechanical properties, several attempts such as refining of the graphite nodules of the alloying of special elements have been made, but not succeeded yet in obtaining sufficient results, besides, these processes may have such disadvantages as requiring of special melting proesses or expensive raw materials.
We have presented in our previous U.S. patent application Ser. No. 583,877, a specific ductile cast iron and its heat treatment method, which may improve such disadvantages as mentioned above.
However, its heat treatment process is rather complicated and may have some disadvantages of requiring special equipments.
Namely, that ductile cast iron of high tensile strength and elongation presented previously is characterized by a fine grained matrix structure having very fine cementite particles dispersed in ferrite grains, and is obtained by an application of 6-10 times repeated thermal cycles each composed of rapid heating to the austenite temperature range and air cooling from that temperature range.
Therefore, operation is somewhat complicated and heat treating bath of such as molten salt, lead or aluminum is required for performance.